Most to least intelligent: Characters from the show.
This is my own personal list/opinion of which of the characters are the most clever and which are the least clever. SHORT DESCRIPTIONS: Edd: Easily the most intelligent member of the cast. This is proven by his insane ability to create super-advanced technology from scratch. He has the most advanced vocabulary, most advanced logic, is the best musical talent, knows the most things, and is pretty much the smartest member of the cast in almost every way. He is also probably the most skilled when it comes to the practices of academia. Kevin: He is very reasonable and is one of the members of the cast that is least likely to fall for the deception of Eds. He has immense dexterity when it comes to sports. He is one of the best leaders of the entire cast. He understand many things. Jimmy: He is very shrewd when it comes to revenge/deception and he is known to have come up with a scam that surpasses the profits of anything Eds have come up with. He is very nutritionally aware. He often thinks of stuff that Double-D has failed to, and even Double-D has called him "ingenious" and "absolutely cunning". Rolf: He is respected by pretty much everybody in the Cul-de-sac (because of this, he is socially intelligent) and is the leader of his own flock of rangers (The Urban Rangers), which are bound by complex traditions that Rolf can comprehend, and they obey Rolf. He is possibly the wealthiest member of the Cul-de-sac when it comes to property. He also has a large vocabulary. He is also known for having incredibly quirky old-world knowledge that has amazed Double-D, and sometimes other members of the cast. Matthew: This is the closest thing to a name for brother. He was a successful scammer (better than any of the Eds) which means he is a shrewd judge of character. He has the ability to construct complex things (such as a giant stink bomb). He can somehow fit an entire car into his room, meaning he is a very skilled engineer of at least SORT. He is also an elder, meaning he knows many things, however, the main reason he ranks so high is that he was a successful scammer, something that Eds, using all of their three brains fail to accomplish. May: She has been shown to have a fine understanding of chemistry in the day special. She is probably the smartest out of the Kanker Sisters because of this. The other Kankers don't have much intellectual merit. Lee: She is the most natural leader of the Kanker Sisters, meaning she is respected among them. Due to the fact that Lee is most respected among her sisters, she ranks second-smartest of the Kankers. Marie: Because Marie is neither a leader nor has she shown any intellectual merit (other than being attracted to Double-D, who is the most intelligent) she naturally fits as being the least intelligent Kanker Sister. Nazz: She has shown to have some understanding in regard to act regulations, but other than that, she has done no clever thing throughout the entire series. She has thrown a box at Sarah, thinking it would be useful as an attack. She lacks ambition and direction in life. Eddy: It may bother many people that Eddy is so low on this list, but the reasons cannot be ignored. Eddy has proven that he is not a clever person. He thought that Rolf possessed an actual seed and traded all of his belongings for it (and he believed in the existence of the money-tree seed even after he was scammed one time in exchange for it, and not even Ed believed that the seed was real the second time, saying "even I am not that dumb, Double-D"). On one occasion, when it was his goal to make Double-D fall into a hole and become trapped, he blamed the failure of the hole on the idea that "it wasn't deep enough" when in reality, the problem with the hole was that the hole didn't have a significant surface area (Double-D simply walked around the hole). He performs poorly in school and thus lacks academic prowess. He is known to be highly delusional and irresponsible. Sarah: She is among the most immature-minded people in the cast. She ranks very closely with Eddy in regard to intelligence, but falls shorter because she is highly destructive to her own property. Eddy has never purposely damaged his own home without merit (he did so in the movie in order to escape the kids, and he did so to trade for a "money-tree seed"), but Sarah has been known to be destructive to her own property just for kicks. Other than all of that, she's basically on-par with Eddy (she is just as rotten), but at least Eddy has serious ambitions (making money), Sarah does not have ambitions. Jonny: He believes inanimate objects with drawn-faces are like people. He speaks to inanimate objects very often throughout the series. Perhaps he has some kind of a savant-syndrome as result of saying semi-intelligent things that he credits to inanimate objects (like Plank), but if we discard this syndrome, he is a fool for believing inanimate objects can communicate just as people do. Ed: This was highly predictable. Ed is lost in his own world, always doing his own separate thing (this isn't his worst trait). He fails to reasonably perform even the simplest of calculations (such as using an entire day or more of thinking in order to come to the conclusion that he cannot share two Slovak-jawbreakers among the trio because two wholes doesn't go into three wholes). He has destroyed many valuable things. You guys know the drill: Ed understand the least out of anybody in the entire series.